In this paper the boron contents and boron isotopic composition of acid-soluble phases in loess and paleosol samples are determined for the first time. The boron contents of acid-soluble phases in the Luochuan loess section (S-0-S-2) vary within the range of (0.8-2.7)x10(-6) and their delta(11)B values vary from -1.8parts per thousand. to +18.6parts per thousand, mostly within the range of 0-+10parts per thousand. The boron contents and delta(11)B values of paleosol layers are higher than those of loess layers, especially in the loess layer S-1. Varying chemical weathering intensity and loess adsorption capability are the main factors leading to the variations of boron contents and delta(11)B values of acid-soluble phases in the loess section. The variation of chemical weathering intensity in response to the variation of climatic conditions seems to be the main factor leading to the variations of boron contents and delta(11)B values of acid-soluble phases in the loess section.